Get the pid first: $ pgrep compton
or maybe but doubtful $ pgrep picom
Then kill it softly: $kill -15 (above returned pid #)
Finally relaunch picom: $picom -b
Also I would make my life easier and replace all the compton calls I could find. the_silver_searcher
will do great here. (and also show you all the compton errors in '.xsession-errors')
$ sudo pacman -S the_silver_searcher
$ cd ~
$ ag compton --depth=0 --hidden
This will show you everywhere compton appears and may need to be edited to picom. (This could be scripted and fed into a loop with sed search and replace command to do it automagically as well)